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CHARACTERS   OF   RODENTS

459

Some, however, such as the Voles, are aquatic; others, e.g. the
Squirrels, are arboreal, and there are " flying " Rodents exemplified
by the genus A-nomcdurus. Their range of habitat is in fact as
wide as that of any other Order of mammals, and wider than
that of, most.

The most distinct anatomical characteristic of the Bodents
concerns the teeth. They are without exception entirely deprived
of canines. Thus there is a long diasteina between the incisors
and the molars. Another peculiarity is, that in many cases the
dentition is absolutely
monophyodont. In
such forms as the
Muridae there seems to
be no milk dentition
at alL In that family
there are only three
molars; but in other
types where there are
four, five, or six molars,
the first one, two, or
three, as the case may
be, have milk pre-
decessors, and may thus
be termed premolars.
This has been definitely
proved to be the case
in the common Habbit, which has the unusually large number of six
grinding teeth in each half of the upper jaw when adult. The
first three of these have milk forerunners. On the other hand the
existence of four molars does not apparently always argue that
the first is a premolar; for Sir W. Flower found that in Hydro-
choeriix?- none of the teeth had any forerunners, at any rate so
far as could be detected from the examination of a very young
animal. The Rabbit appears to be also exceptional, in that the
second incisor of the upper jaw and the incisor of the lower jaw
have milk forerunners. In any case the tendency towards moiio-
phyodontism is peculiarly well-marked in this group of mammals.
The incisors of Rodents are as a rule in each jaw a single pair of
long and strong teeth, which grow from persistent pulps, and

1 Proc. ZooL Soc, 1884, p. 252.

FIG. 231.—Side view of skull of Cape Jumping Hare
(Pedetes caper), x f, ASt Alisphenoid ; JSx.O,
exoccipital; Fr> frontal; X, lachrymal; Ma,
molar; Jfcfas, maxilla; JVa, nasal; O/S, orbito-
sphenoid; Pa, parietal j JPer points to the large
supratympanic or xnastoicL feu-lla ; PMte, pre-
amxilla; Sq, squamosal ; Tyt tympanic. (From
Flower's Osteology.}